This invention relates to image intensifier tubes and more particularly to photoemissive cathodes for use in such tubes.
Image intensifier tubes multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing extraterrestrial bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Image intensifier tubes utilize a photoemissive wafer which is bonded to a glass faceplate to form a cathode. Light enters the faceplate and strikes the wafer, thereby causing a primary emission of electrons.
After the formation of the cathode, a heat cleaning step is performed to remove contaminants, such as oxygen and carbon from the surface of the photoemissive wafer. Bringing the cathode to a specific temperature and maintaining the cathode at that temperature are necessary in effecting proper heat cleaning of the cathode so that its structure and properties are not adversely affected. Knowing the heat cleaning temperature is also necessary in order to avoid the formation of brush lines in the otherwise transparent photoemissive wafer.
Thermocouples cannot be used to measure the temperature of cathodes because they tend to damage the fragile surface of the cathode or give inaccurate readings. The most convenient method is radiative measurement of the temperature using a thermometer based on detection of blackbody radiation.
Instruments which detect infrared radiation a wavelengths for which the layers are transparent and the glass is opaque sense the radiation from a thin layer at the interface of the glass and wafer plus a contribution from the wafer. Due to the proximity of the interface to the gallium arsenide layer and the good thermal conductivity of semiconductors, the measured temperature is a good indicator of the true wafer temperature. However, the wafer acts as a thin film which causes the apparent temperature to vary by a large amount due to interference effects caused by multiple reflection of the blackbody radiation between the internal surfaces of the wafer. However, by measuring the thickness of the wafer and knowing the indices of refraction, a correction can be made for the interference.
One solution to the problem of accurate photocathode temperature measurement during heat cleaning is given in application Ser. No. 814,132, filed Dec. 27, 1985, entitled "Method of Measuring the Temperature of a Photocathode", in the name of A. Amith.